Is my engine supposed to miss at low RPMs?

hotmustang331 · Mar 7, 2007

I dont know if everyone elses car does this, but if I hold the car at a steady 1500RPMs you can hear the car missing maybe 1-2 times per second.
Get to 3K and it totally goes away....but heck my sisters car had a bad valve and was missing BAD at idle, but anything past 3500 and the engine sounded great.

Problem is that I cant tell if its missing at idle (re cams). Also in 5th gear at a steady pace @ 1500RPMs you can feel the car slightly jerking and hear the engine missing a bit. No engine light or anything....and its been that way since I can remember . Definatly since the cams though.

It would ONLY go away when I put brand new plugs in....and only for about a day and then the miss from 1100-2800RPMs (no load) is back. The low RPM (1000-1500) bucking issue that I (and alot of us cam guys) have could be a result of this miss.

Also, when we pulled my old LTs off for the turbo install...I think it was cylinder # 6 (drivers side 2nd to the front) ....I noticed that exhaust port was noticably different from all the others. It was CLEAN...like almost bare aluminum/white looking. No black stuff at all in the inside that you could wipe off like the other exhaust ports....whipe your finger in it and you get nothing, all the others gave you a black finger.
What would cause this? And all the plugs looked about even.

Thanks for any and all comments!

rebelleader · Mar 7, 2007

I have the exact same problem you are describing and have posted about it in the past. Noone had anything useful to offer. I have an Auto, so we can eliminate the Trans as a source of the problem. I also have Stock Exhaust Manifolds, stock heads and Stock cams. Those can all be removed from the discussion as well.

kid rock · Mar 7, 2007

im gonna assumue your plugs, leads and coil packs are ok??

RoadconeTuning · Mar 7, 2007

i think its just fords crappy tune...

mine bucks and crap under 2k if i lift off the throttle... over 2k its fine since ive got my predator on it.

kid rock · Mar 7, 2007

yeah but his is a twin turbo setup, i hope hes not running a standard ford tune lol!

But maybe get your tune looked at by your tuner!

Si

kid rock · Mar 7, 2007

the whole 6th cylinder thing is interesting tho?

tomustang · Mar 7, 2007

what kind of fuel are you running?

01BlkGT · Mar 7, 2007

Got the same thing and I have Tim's tune... it's annoying but I've learned to ignore it.

rebelleader · Mar 7, 2007

Damn, never heard anyone talk about this before.

It happens on my Stock Tune, as well as with my XCAL2 and the MustangTuning Tune.

thomas91169 · Mar 7, 2007

i just checked something out for you, i read this and then left work to get some coffee. held the rpm's at 1500 and it was constant, no miss or hesitation could be heard from the exhaust (especially with no cats, x-pipe and side exhaust).

id do a compression check. if thats good, id then run some seafoam through it, and then change out the plugs once more.

Dusstbuster · Mar 7, 2007

doesnt that mean its running lean?? the clean cylinder that is...

WillDuhhBeast · Mar 7, 2007

I've got the same missing feeling when i gradually go through the gears. It's been the same since it was bone stock. Now it has a new tune, supercharged and nearly fully bolted on and it still has that missing/slight jerky feeling.

I just took it as normal.

SilverBullet00 · Mar 7, 2007

I had a similar problem with the other car that had cams in it. When I ripped it apart to fix the head gasket I had the same issue with that same cylinder- it appeared to be whiter than the rest. My simple solution to the whole bad idle / whiteness issue was to get the car stolen.... I don't recommend this technique.

LaserRed01GT · Mar 7, 2007

Sounds like a COP is weak or going bad. That would cause that cylinder to intermittently run lean and miss. That would also explain the washed down exhaust port on that cylinder.

hotmustang331 · Mar 7, 2007

Thanks for all the comments guys .

Reading through these, it seems that its more or less a random problem...and not so much mod specific. I was also running a .054 plug gap before we pulled the car down...and now I have a .035 gap to prevent spark blowout when I go to the dyno....
not much difference, car still misses....actually it seems worse now.

And I was hopeing you would chime in Laser . I was confused as to what would cause the clean exhaust port...SUPER lean or that cycliner not firing.
I was leaning toward lean, bucause I figured my plug would be fouled if it werent firing....but would an intermentent missfire cause that washing, yet keep the plug looking good?

I wonder if a KB boost-a-spark would help? Im sure it would give me some power regaurdless, but I wonder if it would fix the issue

LaserRed01GT · Mar 7, 2007

Yes, it could wash down the carbon off the exhaust port and inside of the header primary on that cylinder if the COP was intermittently missing. BTW, I meant to say it would intermittently run rich, not lean...I think you know what I meant, my fingers don't always type what I think...or is it I don't always think what I type, now I'm confused.

Seriously, if it was an intermittent problem with the COP the plug could look relatively normal because their would be just enough normal combustion to make it look right. Most of these intermittent miss problems with these at idle or low load is related to a bad COP or vacuum leak and unfortunately most of the time they aren't severe enough to set a specific cylinder misfire code.

hotmustang331 · Mar 7, 2007

Thanks laser! I think im going to put a tester on it and fire the car up and see how much power the coil is putting out....anyone know what voltage these things should be making?

If its dropping off, we may have found a fix to this annoying issue . Just go down to your local Ford dealer and buy a new one haha.

LaserRed01GT · Mar 7, 2007

As for the boost-a-spark. I know lot's of guys talk about spark blowout, but there is no such thing as the spark actually being "blown" out. When the spark plug is fired both valves are completely closed, and the piston is about 15-28° BTDC. In other words there is no incoming air charge, and there is not yet any a/f mixture acceleration by means of quench.

As a spark plug is fired(energized by the coil), the voltage ionizes the air between the plug "gap". This establishes an electrical path across this gap that in turn lowers the voltage requirement to fire the plug adequetly, and current is allowed to properly jump (flow) across the gap. This is when you will actually see the spark. What happens is when the cylinder pressures become great enough, it inhibits this ionization between the gap, and significantly increases the voltage (potential) required to jump from the center electrode to the ground electrode (ground strap).

This also depends on the limitations of the stock coils and coil drivers and stuff like your static CR, ignition timing, boost level, AF ratio, and how efficiently the motor will use the boost.

Most of the FI 2V cars that I have tuned didn't even need a BAP until over 16-17 PSI. I think you said your shortblock is stock right? So you won't see enough boost in my opinion to worry about the BAS.

EDIT: I think the stock COP's are rated at something like 28,000 volts. You can get a high voltage multimeter and test the output, but just be careful. Those things can knock the piss out of you. I would just try to replace it and see if that fixes it.

hotmustang331 · Mar 7, 2007

Awsome...saves me some money . Yea everyone iv talked to is always saying that the incoming air/fuel charge is blowing out their spark (20PSI plus people usually...one guy I know started having ignition problems around 24PSI....and @ 29PSI it was so bad that it made the car slower lol) ....but your right, both valves are closed. No air/fuel flowing directly in the path of the plug while its trying to fire.

It would be nice to just change it first and not risk being shocked....but im betting Ford is REALLY proud of those things if you know what I mean. So I definatly want to make sure something is wrong with it.
If so, the darn thing is probably costing me alot of power....if it cant keep up @ low RPMs, im sure it sucks @ WOT.

supergoat · Mar 7, 2007

I have a similar problem since the 93 octane SCT tune. But only if I give it more than 1/2 throttle in 5th un 1500rpm.

If I back off the timing(it's advanced all the way) it goes away. But since I never go more than 1/2 throttle in 5th under 1500rpms anyway I'm just going to leave the tune how it is.

For what it's worth, it did not do it with the stock tune on my car.

Is my engine supposed to miss at low RPMs?

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